This invention relates to a process and system for removing impurities from natural gas in a floating environment, such as on a ship. More specifically, the invention relates to an integrated membrane/adsorbent/absorbent system for removal of mercury, hydrogen sulfide, water and carbon dioxide from natural gas on a ship that houses natural gas purification equipment.
In an LNG (Liquefied Natural Gas) plant where natural gas is cooled to a very low temperature of about −160° C., which is the normal boiling point of methane, carbon dioxide content in the feed gas stream must be reduced to less than 50 ppmv before liquefaction to avoid formation of dry ice within the system. Commercially this can be achieved by using a physical or chemical solvent absorption process such as contacting the natural gas with an amine solvent to remove the carbon dioxide, which is then followed with the natural gas being sent through a molecular sieve unit for dehydration.
There has been a renewed interest in floating liquefied natural gas (FLNG) systems as a way to develop stranded gas fields, isolated and remote from land. These fields generally are too small for permanent platform installation. An FLNG system will use a ship or barge to house necessary recovery, gas treatment, liquefaction and offloading equipment. Compared to a land based LNG plant, an FLNG system will have a greater need for a modular design to minimize the equipment footprint and weight. An additional challenge for FLNG systems is the effect of sea motion on the performance of processing equipment, especially for systems containing liquid. The removal of carbon dioxide by use of an amine system can be impacted by a loss of efficiency and reliability from rocking and tilting of the column internal components. While both membrane and TSA systems have been used commercially on offshore fixed and floating platforms, no operating experience exists for amine systems in FLNG service.
Depending on the amount of carbon dioxide and the volume of the inlet gas stream, membrane processes have been used to remove the bulk of the carbon dioxide in front of a downstream amine unit followed by molecular sieve adsorbents. One of the benefits of a membrane-amine-adsorbent hybrid system is the reduction of the size of amine system that is needed and as well as a reduction in its energy consumption. Adsorption systems have also been used for front-end feed purification for LNG plants. TSA (Temperature Swing Adsorption) processes employing molecular sieves such as 4A or 13X zeolites can remove both carbon dioxide and water from natural gas streams. A growing application for a TSA process is for peak shaving of pipeline gas, where a portion of the pipeline gas is converted and stored as LNG when demand is low. In the TSA process, the adsorbed carbon dioxide and water in the molecular sieve column are regenerated using a hot purge gas, typically from the feed or the product gas stream. The hot regeneration gas is cooled to knock out most of the water and is then returned to the pipeline. The carbon dioxide removed from the adsorbent, which is not condensable at the cooler temperature, is also returned to the pipeline.
In general, membrane processes that use carbon dioxide-selective polymers such as cellulose acetate can not economically generate a residue or product stream that meets the specification levels of less than 50 ppmv CO2, as the process is limited by the driving force or the CO2 partial pressure across the membrane. Molecular sieve TSA processes typically can not economically handle a feed stream with more than 3% CO2, since the required size of the adsorbent beds become too large and the necessary regeneration gas flow then becomes prohibitively large. Furthermore, for an FLNG application, there is no existing solution to treat or recycle the effluent regeneration gas, which contains the CO2 removed from the feed stream.
Another impurity that requires removal is hydrogen sulfide and more generally other sulfur compounds that may be found in a natural gas stream. When a membrane system is used to remove carbon dioxide from natural gas, the permeate that contains carbon dioxide may be burned as fuel gas or blended into a fuel gas stream. However, if the untreated natural gas contains hydrogen sulfide, the permeate that passes through the membrane will have higher concentrations of hydrogen sulfide. The concentration of hydrogen sulfide in the permeate stream may render it unsuitable for fuel gas due to environmental concerns and other considerations. A hybrid system may be considered unsuitable for H2S containing natural gas. An alternative may be a very large solvent based purification unit that would not be feasible to use when there are stringent limitations on space and weight allocated to the purification process.
In order to use a hybrid purification scheme, the hydrogen sulfide concentration in the membrane permeate stream needs to be significantly reduced. The present invention provides the use of a solid adsorbent bed to remove a majority of the hydrogen sulfide from the gas before it is routed to the membrane separation unit. Then the permeate product will have an acceptable amount of hydrogen sulfide and other sulfur compounds for use in fuel gas.
Frequently, natural gas is contaminated with mercury. In the processing of natural gas for LNG applications, it is necessary to reduce the level of mercury in the natural gas to very low levels. In previous systems, activated carbon beds have been used for removal of mercury. However, activated carbon beds are only usable on streams of purified and dried gas. Many current offshore LNG applications are located in parts of the world where there are high levels of mercury in natural gas. When natural gas with elevated levels of mercury needs to pass through the rest of the purification process before being removed in conventional activated carbon beds, there is a risk that the mercury may condense and increase the risk of failure to welding seams. Also, there is an increased risk that mercury content will increase the hazard levels for equipment entry during maintenance. Activated carbon beds require relatively frequent replacements.
There exists a need to develop an improved process or integrated processes that can remove carbon dioxide and moisture to meet FLNG requirements. The desired processes should be compact and robust, and not susceptible to producing natural gas that is below specification due to winds and waves.